Atomic absorption analysis represents a recently introduced system for the qualitative and quantitative determination of an element in a sample to be investigated on the basis of the specific atomic absorption of the element in question. The atomic absorption method is based on the physical discovery that free atoms contained in atomic vapor existing in the ground state, of an element which is to be detected, absorb only electromagnentic radiation which corresponds to the quantum energy uptake from the ground state to a higher energy condition.
Known atomic absorption spectral devices comprise as primary light sources hollow cathode lamps or other line emission sources which emit an atomic spectrum in a form of very narrow lines of the element or elements forming the cathode; furthermore, there is a device for converting the sample into atomic vapor and a wavelength separating device for selecting the resonance frequency light from the radiation emitted by the light source, as well as a detector device. With the help of optical projecting means the radiation emitted by the hollow cathode or the line emission source is passed through the atomic vapor and supplied via a wavelength separating device to the detector. The absorbed quantity of radiation measured at the resonance frequency line of the element in question is a measure of its concentration.
For converting the sample into an atomic vapor a proposal has already been made to atomize the sample to be analyzed either in a flame (flame atomic absorption method) or to heat the sample in any suitable manner thermally and to convert it into the vapor state (flameless atomic absorption method).
Furthermore it is also known that the specific absorption measured at the resonance frequency, and which forms the actual criterion for the quantitative determination of the element to be detected, has superimposed on it a nonspecific absorption, which is caused by effects such as absorption of the flame, molecular bands in the flame, absorption of the solvent, absorption of the matrix salt or dispersion by the solids present in concentrated solutions, so that the accuracy of the atomic absorption measurement is impaired.
In order to compensate for the nonspecific absorption, arrangements have already been proposed for atomic absorption spectral equipment, having the following parts: an auxiliary radiation source producing radiation within a wavelength range which comprises the resonance wavelength of the resonance radiation produced by the primary light source, and which is broad in comparison with the band width of this resonance radiation of the primary light source; a wavelength selection device which is arranged between the radiation sources and the radiation detector and serves for separating out a narrow wavelength band including the resonance wavelength of the hollow cathode lamp or line emission source; a device by means of which alternate pulses of the resonance radiation provided by the primary light source and of the radiation produced by the auxiliary radiation source are supplied along a reference and a sample ray path to the radiation detector; and circuit means responsive to the electric output signal of the radiation detector to produce an electrical signal having an amplitude which depends only upon the absorption due to the free atoms of the element to be analyzed and which is free of the interfering nonspecific absorption.
In the case of the prior art atomic absorption spectral equipment, as optical auxiliary means for reflecting the continuum radiation emitted by the auxiliary radiation source into the optical equipment systems, use is made of rotating mirror sectors or stationary semi-transparent mirrors, and the matching of the lamp energy of the primary light source and the auxiliary radiation source is carried out via iris diaphragms and/or gray wedges.
The use of rotating mirror sectors involves the disadvantages that on the one hand in the case of a rocking movement the optical identity of the ray paths of the primary and auxiliary ray sources in the absorption vessel is no longer guaranteed and on the other hand the measurement frequency is limited by the speed of rotation of the mirror sector arrangement whereby rapid compensation of the nonspecific absorption, as is required more particularly for the purposes of flameless absorption methods, is not possible.
The use of semi-transparent mirrors is disadvantageous in as far as their use involves substantial losses in energy and an offsetting of the ray which is dependent on the thickness, and, especially as to date it has not been possible to produce a dividing mirror in the UV-spectral range which is of interest for the absorption method.